Carbon nanotubes (CNTs) have been attracting attentions in recent years as new materials for nanotechnologies (Non-patent Literature 1). Among these, single-walled carbon nanotubes (SWCNTs) are expected to be applied to various fields, owing to their simple structures and specific physical and chemical properties.
However, due to the association (bundling) by the strong van der Waals interaction of the CNTs themselves, it is quite difficult to solubilize or disperse CNTs in solvents, which is a conspicuous hindrance for the development and application of materials.
Hitherto, various studies have been made chemically or physically with respect to methods of solubilizing CNTs in solvents. Proposals include a technique for forming a functional group that enhance the solubility of a CNT in a solvent by ultrasonification of the CNT in an acidic solution (Non-patent Literature 2), and a technique for accelerating dispersion (solubilization) in a solvent by mixing with a dispersant. The dispersants reported include ionic amphipathic compounds, compounds having an aromatic functional group, naturally-occurring polymers, synthetic polymers, and the like (Patent Literature 1). However, in many cases of them, there is no alternative but to remove the dispersant that has been adsorbed on the CNT by washing for a long time period, calcination, or removing by decomposition by an agent. Thus, it is still required to develop a dispersant that can be readily removed for taking out a pure CNT. Furthermore, it is also important in view of resource saving, to develop a dispersant having recyclability that can be utilized repeatedly, by repeatedly controlling the dispersibility as necessary, and causing reaggregation of the dispersed CNT to collect the same appropriately.
Until now, several examples are known, in which the dispersibility was controlled, by changing the structure and solubility of a dispersant by any conditions. For example, in Non-patent Literature 3, a CNT is dispersed in the form of a micelle, by using an amphipathic compound, which is malachite green, as a dye, substituted to have polyethylene glycol, and the thus-dispersed CNT is caused reaggregation, by utilizing the change in solubility by photodecomposition ionization of the malachite green by light irradiation. Furthermore, in Patent Literature 2, a CNT is dispersed, by using an amphipathic oligopeptide as a dispersant, and then only the dispersant is decomposed biochemically by using a protease, to isolate the CNT precipitated. On the other hand, in Non-patent Literature 4 and Patent Literature 3, a dispersant composed of a metal complex is synthesized, and using this, the dispersibility is suitably changed by controlling the affinity to a CNT, by utilizing the change in conformation (=the change in molecular structure) by chemical oxidation/reduction of the central metal.
However, problems to be solved still remain in either case of above. For example, the dispersant derived from malachite green in Non-patent Literature 3 requires an addition amount of ten times greater than that of the CNT on the basis of weight ratio, and thus it cannot be considered that the CNT is dispersed efficiently. Furthermore, in the method of collecting a pure CNT by the biochemical decomposition of the dispersant in Patent Literature 2, it is difficult to collect and reuse the dispersant, since the utilization is limited to only under a condition in which the enzyme exhibits the activity due to its principle, and the decomposition is irreversible. In Non-patent Literature 4 and Patent Literature 3, although dispersion and aggregation can be controlled reversibly, a high-output ultrasonic irradiation method and a high-speed vibration milling method that requires special apparatus are used in combination so as to solubilize the CNT, and thus the manner for preparing the dispersion cannot be considered to be convenient. Furthermore, since chloroform, an amide-based organic solvent, or the like is used as a dispersion medium, environmental load is concerned when a condition in which the dispersion medium is used industrially in a large amount is taken into consideration.    Patent Literature 1: JP-A-2004-2850 (“JP-A” means unexamined published Japanese patent application) (published date: Jan. 8, 2004)    Patent Literature 2: JP-A-2007-153716 (published date: Jun. 21, 2007)    Patent Literature 3: JP-A-2009-23886 (published date: Feb. 5, 2009)    Non-Patent Literature 1: S. Iijima, Nature, 354, 56 (1991), S. Iijima, T. Ichihashi, Nature, 363, 603 (1993)    Non-Patent Literature 2: J. Chen, et al, Science, 282, 95 (1998)    Non-Patent Literature 3: S. Chen, et al., Langmuir, 24, 9233 (2008)    Non-Patent Literature 4: K. Nobusawa et al., Angew. Chem. Int. Ed., 47, 4577 (2008)